Cathode ray picture tube



March 27, 1956 L. E. SWEDLUND CATHODE RAY PICTURE TUBE Filed May 27, 1952 INVENTOR LLUYD E.WEDLUND ATTORNEY CATHODE RAY PICTURE TUBE Lloyd Edward Swedlund, Lancaster, Pa., assignor to Radio Corporation of America, a corporation of Delaware Application May 27, 1952, Serial No, 290,151

4 Claims. (Cl. 313-..6.4)

My invention is directed to electron discharge tubes, and more particularly to cathode ray picture tubes used for viewing of television pictures.

The usual television picture tube is one consisting of an evacuated tubular envelope having a large conical or frusto-pyramidal tubular portion, which is closed at its large end by a face plate of glass. The smaller end of the cone normally has fixed thereto a tubular glass portion, which is used as a housing or support for an electron gun. The inner surface of the glass face plate is covered with a phosphor film known as the fluorescent screen which produces a luminesce when struck by electrons from the electron gun. The electron emission of a cathode in the gun is formed into an electron beam which is brought to a sharp point of focus on the surface of the fluorescent screen. Normally, magnetic fields are used to scan the electron beam over the surface of the fluorescent screen to form a rectangular shape raster. Incoming video signals are applied between the cathode and control grid of the electron gun to modulate the electron beam so that the luminesce produced on the fluorescent screen varies from point to point and produces a light distribution or picture on the fluorescent screen.

The trend in television tube development is one directed toward larger picture tubes requiring large evacuated containers for housing the electron gun and the large fluorescent screen. One form of large tube development has been that of utilizing metal containers or shells, to

which the face plate and the tubular gun housing are sealed. The use of metal shells has the advantage of providing greater over-all strength to withstand the high stresses due to atmospheric pressure. Also the use of a metal shell enables the tube to have a greater safety factor in case of implosion. If the metal of the shell can be made suiiiciently thin, an added advantage is the fact that metal tubes have inherently less weight than comparable glass tubes, for example.

Furthermore, in making large metal shells, it is desirable to use a minimum amount of metal and one of low cost. The development .of large metal shells for television picture tubes has progressed from the use of high chromium content iro n alloys to the use of low cost, low carbon steels, sometimes known as irons and cold rolled ,steel. Many problems arise in the use of such steels as the metal shell portion of a television picture tube. For example, special glasses for the face plate and the neck portion are required, which not only will seal to the iron but also which will have the proper expansion characteristics to match that of the metal. Furthermore, in forming the seal, there are additional problems, as for example, the over-oxidation of the metal rim in the sealing process, which in turn produces a porous leaky glass-to-metal seal. Furthermore, normal processing of low carbon steel tubes results in oxidation or corrosion of the metal surfaces, which produces scaling. The scaling in turn is objectionable, not only from the fact that it gives the shell a bad appearance but more important that in-the-formation of the-fluorescent screen,

States Patent preventing such light "refle scale particles produce holes and tears in the fluorescent layer. While the use of some protective coating on the metal might suggest itself, the problem can be solved only by providing aeoatihg which will not deteriorate during sealing and processing and which will not interfere with the sealing and processing operations. The coating must also be one which will lQt interfere with the operation of the device when inuse. It must also be one which can be readily applied. I

Furthermore, the use of a metal shell as a television picture tube envelope results in an inherently smooth and somewhat shiny surface on the interior of the enyelope adjacent to the fluorescent screen, and results'in "considerable light from the screen beinglreflect'ed back onto the screen from the metal surface; This reflected light falls on the dark as well as the bright parts of the picture on the screen and thus acts to reduce thefpicture 'contrast. It is desirable in any picture tube to have high contrast between the black and white portions of the fluorescent screen. Several attempts have been made to improve the contrast of the picture in a metal tube by eliminating light reflections from internal surfaces. These attempts have included such procedures as painting the inner metal surface with a black graphite coating, 'as used in glass envelopes. Such a coating, however, is difficult to apply to the metal cone, particularly close to the fluorescent screen. it is also a diflicult coating to remove if the cone is'to be re-run. Attempts have also been made to darken the inner'surface of the metal tube by chemical treatment. Applicant believed that thesefattempts have not been considered satisfactory as they are difiicult to apply and are not sufiiciently light absorbent.

From the above, therefore, it is clear thatlalthough glasses, which will satisfactorily seal to metalfeithier chrome-iron or steel, ,can be provided, severe problems still remain. It is necessary to prevent over-oxidation and corrosion of the metal cone from occurring during processing with the resulting bad appearance of the steel and adverse effects on the phosphor screen and yetat the same time normal sealing" and other processing procedures must be permitted." It is also necessary to provide means for preventing light reflections from the interior of the metal cone'jwhicli means is not subject ,to the deficiencies of those means previously employedfer ctions in tubes having metal envelopes.

It is, therefore, a principal .object of my invention to provide a television picture tube having'a rnetal'shell and which nevertheless ,can be processed andtsealed' b y conventional techniques and withoutthe usual corrosion and oxidation of the metal cone accornpanying'such' op erations. i i i More specifically, it is an object of my invention to provide a television picture tube'having an traitor steel metal shell to which the glass portions of .the'envelope may be sealed without over-oxidation of the exp'osed metal shell portions. i

A further object of my invention is to provide a television picture tube having aniron or steel metal shell which can be subjected to the usual processingtechniques without causing corrosion or oxidation-producing:scaling of the exposed metal shell portions thereby insuring a good appearance of the tube and a uniform phosphor screen without holes.

It is another object of my invention to provide a metal shell for a television picture tube, which will minimize light reflection from the fluorescent screen within the tube In accordance with my invention I have developed a coating and a technique of coating which provides a solution for the problems enumerated above.

My invention involves the use of glass frit or glaze coating on the surface of the metal shell to be used for a television picture tube. The glaze or glass frit may be coated over that portion only near the face plate but I prefer to coat the entire inside surface of the cone as well as the outside. The coating is selected to give a dull, high light-absorptive surface which may be produced by mixing infusible dark colored material in the glass frit. This produces a dull or mat surface which minimizes light reflection within the tube envelope during tube operation.

The single figure shows a partially cut away view in perspective of a television picture tube made in accordance with the invention.

The television picture tube shown consists of a frustopyramidal portion closed at its large end by a glass sheet or face plate 12 and at its smaller end by a glass tubular portion 14, which is flared at one end, as is shown at 16 and sealed to the metal sheell 10. The inner surface of the glass face plate 12 is coated with a fluorescent film 18 of a phosphor material. Such phosphors are those which upon bombardment by electrons luminesce with a substantially white light. Within the neck portion 14 of the envelope, there is mounted an electron gun 20 consisting of a source of electrons or cathode electrode 22, housed back of a control grid 24. Spaced from the grid 24, are accelerating electrodes 26 and 28 for accelerating and forming the electrons emitted from cathode 22 into an electron beam directed at the phosphor screen 18. The electron beam is focussed to a fine spot on the fluorescent screen 18 by a magnetic field produced by a focussing magnet 30 arranged around the neck of the tube.

The details of the gun structure and its operation are well known and do not constitute a part of this invention.

The electron beam, produced by gun 20 during tube operation, is normally scanned in a substantially rectangular raster over the inner surface of the fluorescent screen 18 by magnetic fields produced by coils within a yoke member 32 around the neck 14 of the tube envelope. These scanning coils are conventional and well known.

Incoming television signals are normally applied between control grid 24 and cathode 22 of the electron gun to provide a modulated electron beam. As the modulated beam is scanned over the surface of screen 18, it will produce a pattern of light as the picture of the tube. Since the fluorescent screen is normally insulating, the electrons striking the screen are dissipated by secondary electron emission to the side walls of the tube envelope.

However, during operation of the tube, there is a loss of contrast between the light portions and black portions of the picture due to the reflection of light from the inner surface of the metal shell 10. This reflected light causes the dark or black portions of the picture to be brightened relatively much more than the white portions.

Contrast is expressed by the ratio of the brightness of the whitest area to the brightness of the blackest area. What is known as large area contrast is particularly affected by internal bulb reflections. Large area contrast is measured by scanning only half of the screen 18 with an unmodulated beam, so that there is produced on this half of the screen a bright area. The contrast is measured as the ratio of the brightness in the center of the scanned half of the raster to the brightness at the center of the unscanned half of the raster. A high contrast ratio is neces sary to produce What photographers call brilliance and good separation of the gray tones between black and white. For a tube, in which the internal light reflection is low, the contrast ratio is in the order of 60 to 1. However, for metal cones having a metal shell, and in which there is no special coating or treatment of the inner surface of the metal shell, the contrast. ratio is in the order of 30 to 1. Glass picture tubes, in which the glass envelope is coated by painting the inner glass surface with a colloidal solution of graphite produce low light reflection, and have a contrast ratio in the order of 60 to l.

In accordance with my invention the metal shell 10 of a television picture tube is provided with a coating 34 of a glass frit or glaze. To minimize light reflection, I mix an infusible dark colored material such as a powder in the glass frit. This produces a dull mat-surface coating over the metal shell 10. To minimize light reflection within the metal shell during tube operation, I apply the dark mat frit or glaze to the metal surface adjacent the fluorescent screen 18. Preferably, however, I coat the entire inside of the shell 10 as shown in the figure. And as further shown in the figure, the dark glaze coating may be extended over the entire surface of the metal shell both inside and out.

The black enamel coating 34 may be formed from any appropriate material. However, the following composition is one which has been successfully used to coat the inner surface of metal shells used in television picture tubes. A mixture is made up of parts of a vitreous enamel or glass frit, 25 parts of calcined alumina, 10 parts of clay and 40 parts of water. The materials are thoroughly mixed together and 8 parts of a mixture of black copper and chromium oxides are added to provide a dark color to the glass frit or porcelain enamel.

The mixture may be applied to the metal shell 10 by either spraying the material or dipping the shell into the mixture. The coated shell is then fired at a temperature somewhere between 860 C. and 871 C. for a period of time in the order of 5 to 10 minutes. The firing of the enamel coating causes it to adhere tightly to the metal shell 10. The result is an enamel coating from 15 to 20 mils thick over the surface of the metal shell. The coating provides a dark mat surface which provides a minimum of light reflection from the inner surface of the shell, during tube operation.

Picture tubes made from metal shells which have been coated by this dark mat porcelain enamel have a contrast ratio in the order of 60 to l. The use of the black mat enamel coating, thus provides twice the contrast as a metal shell without the coating. Furthermore, the characteristics of the enamel coating described and the thinness with which it is applied to the metal shell 10 provides sufficient electrical conductivity through the coating to the metal shell to prevent the storage on its surface of secondary electrons from the fluorescent screen, during tube operation. If the conductivity of the enamel coating used is insuflicient to conduct to the metal shell the secondary electrons which leave the fluorescent screen, it is desirable to coat the inner portion of the metal surface sufficiently thin and in a manner to leave the metal uncoated in many small areas or spots. This permits conduction to the metal cone of the secondary electrons from the screen. Thus, the inner surface of the metal shell 10 does not become negatively charged by electron collection, which would affect scansion of the beam and distort the picture. In practising such spot coating, the coating is well broken up to present a dark mat appearance and to reduce light reflection.

The coating described above enables the use of low cost, low carbon steels to form the shells of picture tubes. Such low carbon steel shells are inherently subject to corrosion and overoxidation during such tube processes as the sealing of the glass face plate to the metal shell and in the formation of the luminescent screen 18 on the face plate of the envelope. Unless such corrosion of the metal can be greatly minimized or entirely eliminated, the use of such low cost steels for metal kinescopes is impracticable. By coating a metal shell 10 with a glass-enamel in accordance with my invention, the metal shell is so protected that during tube processing little or no corrosion or over-oxidation occurs. Furthermore, if the metal shell is coated before the glass face plate is sealed to the a shell, the seal region between shell and face plate 12 is sufliciently protected that over-oxidation of the metal surface does not occur during face plate sealing. This eliminates the formation of spongy and porous seals be tween the glass and metal. Such seals are structurally coating on the inner surface of said ferric shell portion,

weak and can not withstand the compressional forces said coating including an enamel'containing a powdered metal oxide.

2. An electron discharge device comprising, an enve- -"-lope including a ferric shell portion and a glass portion sealed thereto, a phosphor on said glass portion, a mat coating on the inner surface of said ferric shell portion, 7 i said coating including an enamel containing a powdered The screen 18 is normally formed by settling the phosphor film from a suspension of the phosphor in an aqueous solution, which is poured into the envelope. through the tubular portion 14 before the gun is sealed in. Using metal oxide, said enamel coating being sufficiently thin to provide electrical conduction therethrough.

3. A cathode ray tube comprising, an envelope includ- "ing a ferric shell portion and a glass portion sealed thereto, a phosphor on said glass portion, an electron gun mounted within said envelope for directing electrons onto said phosphor screen, a mat coating on the inner surface of said ferric shell portion, said coating including an steel or iron, as the metal portion of the envelope results i in excessive corrosion of the metal surfaces by the solu tions used in the screening process. metal surfaces results in scaling, which falls off on the phosphor surfaces and contaminates the phosphor. screen 18. The enamel coating, however, of the steel shell surfaces eliminates such corrosion and enables the formation of the phosphor screen without undue difiiculty.

It has also been found that the enamel coating, described above, using alumina provides a dark mat porous coating over the surface of the metal cone. Because of the porous nature of the coating, it is possible to degas the tube more completely. Also, the conductivity of the layer is improved, which enables satisfactory elimination from the metal shell surface of secondary electrons from the target. Smooth enamel glaze coatings on the metal shell of a cathode ray tube, appear to trap gases when applied. Such tubes are ditficult to degas at tube exhaust. Trapped gases are later released during tube operation with a resulting low life for the tube.

While certain specific embodiments have been illustrated and described, it will be understood that various changes and modifications may be made therein without departing from the spirit and scope of the invention.

What I claim is:

1. An electron discharge device comprising, an enve lope including a ferric shell portion and a glass portion sealed thereto, a phosphor on said glass portion, a mat Corrosion of the -enamel containing a powdered metal oxide, said coating being sufiiciently thin to provide conduction to said ferric shell portion of electron from said phosphor screen.

4. An electron discharge device comprising, an envelope including a ferric shell portion and a glass portion sealed thereto, a phosphor on said glass portion, an electron gun mounted within said envelope for directing electrons onto said phosphor screen, a mat coating on the inner surface of said ferric shell portion, said coating including an enamel containing a powdered metal oxide, said coating forming a discontinuous film whereby small portions of said ferric shell portion are exposed to the inside of said envelope.

References Cited in the file of this patent UNITED STATES PATENTS 187,886 Milligan et a1. Feb. 27, 1877 906,628 Holley Dec. 15, 1908 1,024,702 Russell Apr. 30, 1912 2,151,992 Schwartz Mar. 28, 1939 2,222,197 Engels Nov. 19, 1940 2,232,098 Deichman Feb. 18, 1941 2,244,245 Ehrenberg June 3, 1941 2,254,090 Power Aug. 26, 1941 FOREIGN PATENTS 488,558 Great Britain July 6, 1938 

1. AN ELECTRON DISCHARGE DEVICE COMPRISING, AN ENVELOPE INCLUDING A FERRIC SHELL PORTION AND A GLASS PORTION SEALED THERETO, A PHOSPHOR ON SAID GLASS PORTION, A MAT COATING ON THE INNER SURFACE OF SAID FERRIC SHELL PORTION, SAID COATING INCLUDING AN ENAMEL CONTAINING A POWDERED METAL OXIDE. 